Targeting Sclerostin in Postmenopausal Osteoporosis: Focus on Romosozumab and Blosozumab

Bone-organ axes: bidirectional crosstalk

In addition to its recognized role in providing structural support, bone plays a crucial role in maintaining the functionality and balance of various organs by secreting specific cytokines (also known as osteokines). This reciprocal influence extends to these organs modulating bone homeostasis and development, although this aspect has yet to be systematically reviewed. This review aims to elucidate this bidirectional crosstalk, with a particular focus on the role of osteokines. Additionally, it presents a unique compilation of evidence highlighting the critical function of extracellular vesicles (EVs) within bone-organ axes for the first time. Moreover, it explores the implications of this crosstalk for designing and implementing bone-on-chips and assembloids, underscoring the importance of comprehending these interactions for advancing physiologically relevant in vitro models. Consequently, this review establishes a robust theoretical foundation for preventing, diagnosing, and treating diseases related to the bone-organ axis from the perspective of cytokines, EVs, hormones, and metabolites.

Platelet factor 4 induces bone loss by inhibiting the integrin α5-FAK-ERK pathway

BACKGROUND

The effect of platelet factor 4 (PF4) on bone marrow mesenchymal stem cells (BMMSCs) and osteoporosis is poorly understood. Therefore, this study aimed to evaluate the effects of PF4-triggered bone destruction in mice and determine the underlying mechanism.

METHODS

First, in vitro cell proliferation and cell cycle of BMMSCs were assessed using a CCK8 assay and flow cytometry, respectively. Osteogenic differentiation was confirmed using staining and quantification of alkaline phosphatase and Alizarin Red S. Next, an osteoporotic mouse model was established by performing bilateral ovariectomy (OVX). Furthermore, the PF4 concentrations were obtained using enzyme-linked immunosorbent assay. The bone microarchitecture of the femur was evaluated using microCT and histological analyses. Finally, the key regulators of osteogenesis and pathways were investigated using quantitative real-time polymerase chain reaction and Western blotting.

RESULTS

Human PF4 widely and moderately decreased the cell proliferation and osteogenic differentiation ability of BMMSCs. Furthermore, the levels of PF4 in the serum and bone marrow were generally increased, whereas bone microarchitecture deteriorated due to OVX. Moreover, in vivo mouse PF4 supplementation triggered bone deterioration of the femur. In addition, several key regulators of osteogenesis were downregulated, and the integrin α5-focal adhesion kinase-extracellular signal-regulated kinase (ITGA5-FAK-ERK) pathway was inhibited due to PF4 supplementation.

CONCLUSIONS

PF4 may be attributed to OVX-induced bone loss triggered by the suppression of bone formation in vivo and alleviate BMMSC osteogenic differentiation by inhibiting the ITGA5-FAK-ERK pathway.

Immunology of osteoporosis: relevance of inflammatory targets for the development of novel interventions

Osteoporosis is recognized as low bone mass and deteriorated bone microarchitecture. It is the leading cause of fractures and consequent morbidity globally. The established pathophysiological evidence favors the endocrine factors for osteoporosis and the role of the immune system on the skeletal system has been recently identified. Due to the common developmental niche bone and immune system interactions have led to the emergence of osteoimmunology. Immune dysregulation can initiate inflammatory conditions that adversely affect bone integrity. The role of immune cells, such as T-lymphocytes subsets (Th17), cannot be neglected in the pathogenesis of osteoporosis. Local inflammation within the bone from any cause attracts immune cells that participate in the activation of osteoclasts. This work summarizes the present knowledge of osteoimmunology in reference to osteoporosis and identifies novel targets for immunotherapy of osteoporosis.

The roles of circRNA-miRNA-mRNA networks in the development and treatment of osteoporosis

Osteoporosis is a systemic metabolic disease, mainly characterized by reduced bone mineral density and destruction of bone tissue microstructure. However, the molecular mechanisms of osteoporosis need further investigation and exploration. Increasing studies have reported that circular RNAs (circRNAs), a novel type of RNA molecule, play crucial roles in various physiological and pathological processes and bone-related diseases. Based on an in-depth understanding of their roles in bone development, we summarized the multiple regulatory roles and underlying mechanisms of circRNA-miRNA-mRNA networks in the treatment of osteoporosis, associated with bone marrow mesenchymal stem cells (BMSCs), osteoblasts, and osteoclasts. Deeper insights into the vital roles of circRNA-miRNA-mRNA networks can provide new directions and insights for developing novel diagnostic biomarkers and therapeutic targets in the treatment of osteoporosis.

An Overview of the Molecular Mechanisms Contributing to Musculoskeletal Disorders in Chronic Liver Disease: Osteoporosis, Sarcopenia, and Osteoporotic Sarcopenia

The prevalence of osteoporosis and sarcopenia is significantly higher in patients with liver disease than in those without liver disease and osteoporosis and sarcopenia negatively influence morbidity and mortality in liver disease, yet these musculoskeletal disorders are frequently overlooked in clinical practice for patients with chronic liver disease. The objective of this review is to provide a comprehensive understanding of the molecular mechanisms of musculoskeletal disorders accompanying the pathogenesis of liver disease. The increased bone resorption through the receptor activator of nuclear factor kappa (RANK)-RANK ligand (RANKL)-osteoprotegerin (OPG) system and upregulation of inflammatory cytokines and decreased bone formation through increased bilirubin and sclerostin and lower insulin-like growth factor-1 are important mechanisms for osteoporosis in patients with liver disease. Sarcopenia is associated with insulin resistance and obesity in non-alcoholic fatty liver disease, whereas hyperammonemia, low amount of branched chain amino acids, and hypogonadism contributes to sarcopenia in liver cirrhosis. The bidirectional crosstalk between muscle and bone through myostatin, irisin, β-aminoisobutyric acid (BAIBA), osteocalcin, as well as the activation of the RANK and the Wnt/β-catenin pathways are associated with osteosarcopenia. The increased understandings for these musculoskeletal disorders would be contributes to the development of effective therapies targeting the pathophysiological mechanism involved.

Knockdown of POSTN Inhibits Osteogenic Differentiation of Mesenchymal Stem Cells From Patients With Steroid-Induced Osteonecrosis

Steroid-induced osteonecrosis of femoral head (SONFH) is a common and serious complication caused by long-term and/or excessive use of glucocorticoids (GCs). The decreased activity and abnormal differentiation of bone marrow mesenchymal stem cells (BMSCs) are considered to be one of the major reasons for the onset and progression of this disease. Periostin (POSTN) is a matricellular protein which plays an important role in regulating osteoblast function and bone formation. Sclerostin (SOST) is a secreted antagonist of Wnt signaling that is mainly expressed in osteocytes to inhibit bone formation. However, the exact role of POSTN and SOST in SONFH has not been reported yet. Therefore, we detected the differential expression of POSTN and SOST in BMSCs of SONFH Group patients, and Control Group was patients with traumatic ONFH (TONFH) and developmental dysplasia of the hip (DDH). Furthermore, we used lentiviral transfection to knockdown POSTN expression in BMSCs of patients with SONFH to study the effect of POSTN knockdown on the SOST expression and osteogenic differentiation of BMSCs. The results indicated that the endogenous expression of POSTN and SOST in BMSCs of SONFH Group was upregulated, compared with Control Group. POSTN was upregulated gradually while SOST was downregulated gradually at days 0, 3, and 7 of osteogenic differentiation of BMSCs in Control Group. Contrarily, POSTN was gradually downregulated while SOST was gradually upregulated during osteogenic differentiation of BMSCs in SONFH Group. This could be due to increased expression of SOST in BMSCs, which was caused by excessive GCs. In turn, the increased expression of POSTN in BMSCs may play a role in antagonizing the continuous rising of SOST during the osteogenic differentiation of BMSCs in patients with SONFH. POSTN knockdown significantly attenuated osteo-specific gene expression, alkaline phosphatase activity, and calcium nodule formation in vitro; thus inhibiting the osteogenic differentiation of BMSCs in patients with SONFH. Besides, POSTN knockdown upregulated SOST expression, increased GSK-3β activity, and downregulated β-catenin. These findings suggest that POSTN have an essential role in regulating the expression of SOST and osteogenic differentiation of BMSCs in patients with SONFH, and POSTN knockdown suppresses osteogenic differentiation by upregulating SOST and partially inactivating Wnt/β-catenin signaling pathway. Therefore, targeting POSTN and SOST may serve as a promising therapeutic target for the prevention and treatment of SONFH.

Evaluating the cardiovascular safety of sclerostin inhibition using evidence from meta-analysis of clinical trials and human genetics

Inhibition of sclerostin is a therapeutic approach to lowering fracture risk in patients with osteoporosis. However, data from phase 3 randomized controlled trials (RCTs) of romosozumab, a first-in-class monoclonal antibody that inhibits sclerostin, suggest an imbalance of serious cardiovascular events, and regulatory agencies have issued marketing authorizations with warnings of cardiovascular disease. Here, we meta-analyze published and unpublished cardiovascular outcome trial data of romosozumab and investigate whether genetic variants that mimic therapeutic inhibition of sclerostin are associated with higher risk of cardiovascular disease. Meta-analysis of up to three RCTs indicated a probable higher risk of cardiovascular events with romosozumab. Scaled to the equivalent dose of romosozumab (210 milligrams per month; 0.09 grams per square centimeter of higher bone mineral density), the SOST genetic variants were associated with lower risk of fracture and osteoporosis (commensurate with the therapeutic effect of romosozumab) and with a higher risk of myocardial infarction and/or coronary revascularization and major adverse cardiovascular events. The same variants were also associated with increased risk of type 2 diabetes mellitus and higher systolic blood pressure and central adiposity. Together, our findings indicate that inhibition of sclerostin may elevate cardiovascular risk, warranting a rigorous evaluation of the cardiovascular safety of romosozumab and other sclerostin inhibitors.

Osteocyte-Mediated Translation of Mechanical Stimuli to Cellular Signaling and Its Role in Bone and Non-bone-Related Clinical Complications

PURPOSE OF REVIEW

Osteocytes comprise > 95% of the cellular component in bone tissue and produce a wide range of cytokines and cellular signaling molecules in response to mechanical stimuli. In this review, we aimed to summarize the molecular mechanisms involved in the osteocyte-mediated translation of mechanical stimuli to cellular signaling, and discuss their role in skeletal (bone) diseases and extra-skeletal (non-bone) clinical complications.

RECENT FINDINGS

Two decades before, osteocytes were assumed as a dormant cells buried in bone matrix. In recent years, emerging evidences have shown that osteocytes are pivotal not only for bone homeostasis but also for vital organ functions such as muscle, kidney, and heart. Osteocyte mechanotransduction regulates osteoblast and osteoclast function and maintains bone homeostasis. Mechanical stimuli modulate the release of osteocyte-derived cytokines, signaling molecules, and extracellular cellular vesicles that regulate not only the surrounding bone cell function and bone homeostasis but also the distant organ function in a paracrine and endocrine fashion. Mechanical loading and unloading modulate the osteocytic release of NO, PGE₂, and ATPs that regulates multiple cellular signaling such as Wnt/β-catenin, RANKL/OPG, BMPs, PTH, IGF1, VEGF, sclerostin, and others. Therefore, the in-depth study of the molecular mechanism of osteocyte mechanotransduction could unravel therapeutic targets for various bone and non-bone-related clinical complications such as osteoporosis, sarcopenia, and cancer metastasis to bone.

The shift in the balance between osteoblastogenesis and adipogenesis of mesenchymal stem cells mediated by glucocorticoid receptor

Mesenchymal stem cells (MSCs) are multipotent cells capable of differentiating into several tissues, such as bone, cartilage, and fat. Glucocorticoids affect a variety of biological processes such as proliferation, differentiation, and apoptosis of various cell types, including osteoblasts, adipocytes, or chondrocytes. Glucocorticoids exert their function by binding to the glucocorticoid receptor (GR). Physiological concentrations of glucocorticoids stimulate osteoblast proliferation and promote osteogenic differentiation of MSCs. However, pharmacological concentrations of glucocorticoids can not only induce apoptosis of osteoblasts and osteocytes but can also reduce proliferation and inhibit the differentiation of osteoprogenitor cells. Several signaling pathways, including the Wnt, TGFβ/BMP superfamily and Notch signaling pathways, transcription factors, post-transcriptional regulators, and other regulators, regulate osteoblastogenesis and adipogenesis of MSCs mediated by GR. These signaling pathways target key transcription factors, such as Runx2 and TAZ for osteogenesis and PPARγ and C/EBPs for adipogenesis. Glucocorticoid-induced osteonecrosis and osteoporosis are caused by various factors including dysfunction of bone marrow MSCs. Transplantation of MSCs is valuable in regenerative medicine for the treatment of osteonecrosis of the femoral head, osteoporosis, osteogenesis imperfecta, and other skeletal disorders. However, the mechanism of inducing MSCs to differentiate toward the osteogenic lineage is the key to an efficient treatment. Thus, a better understanding of the molecular mechanisms behind the imbalance between GR-mediated osteoblastogenesis and adipogenesis of MSCs would not only help us to identify the pathogenic causes of glucocorticoid-induced osteonecrosis and osteoporosis but also promote future clinical applications for stem cell-based tissue engineering and regenerative medicine. Here, we primarily review the signaling mechanisms involved in adipogenesis and osteogenesis mediated by GR and discuss the factors that control the adipo-osteogenic balance.

The genetic architecture of osteoporosis and fracture risk

Osteoporosis and fracture risk are common complex diseases, caused by an interaction of numerous disease susceptibility genes and environmental factors. With the advances in genomic technologies, large-scale genome-wide association studies (GWAS) have been performed which have broadened our understanding of the genetic architecture and biological mechanisms of complex disease. Currently, more than ~90 loci have been found associated with DXA derived bone mineral density (BMD), over ~500 loci with heel estimated BMD and several others with other less widely available bone parameters such as bone geometry, shape, and microarchitecture. Notably, several of the pathways identified by the GWAS efforts correspond to pathways that are currently targeted for the treatment of osteoporosis. Overall, tremendous progress in the field of the genetics of osteoporosis has been achieved with the discovery of WNT16, EN1, DAAM2, and GPC6 among others. Assessment of the function and biological mechanisms of the remaining genes may further untangle the complex genetic landscape of osteoporosis and fracture risk. With this review we aimed to provide a general overview of the existing GWAS studies on osteoporosis traits and fracture risk.

Bone Diseases in Patients with Chronic Liver Disease

Osteoporosis is a frequently observed complication in patients with chronic liver disease, particularly liver cirrhosis and cholestatic liver diseases. In addition, osteoporosis is critical in patients receiving a liver transplant. Nevertheless, few studies have evaluated bone diseases in patients with more frequently observed chronic liver disease, such as chronic viral hepatitis, nonalcoholic fatty liver disease and alcoholic liver disease. Osteoporosis is a disease caused by an imbalance in the activities of osteoblasts and osteoclasts. Over the last few decades, many advances have improved our knowledge of the pathogenesis of osteoporosis. Importantly, activated immune cells affect the progression of osteoporosis, and chronic inflammation may exert an additional effect on the existing pathophysiology of osteoporosis. The microbiota of the intestinal tract may also affect the progression of bone loss in patients with chronic liver disease. Recently, studies regarding the effects of chronic inflammation on dysbiosis in bone diseases have been conducted. However, mechanisms underlying osteoporosis in patients with chronic liver disease are complex and precise mechanisms remain unknown. The following special considerations in patients with chronic liver disease are reviewed: bone diseases in patients who underwent a liver transplant, the association between chronic hepatitis B virus infection treatment and bone diseases, the association between sarcopenia and bone diseases in patients with chronic liver disease, and the association between chronic liver disease and avascular necrosis of the hip. Few guidelines are currently available for the management of low bone mineral density or bone diseases in patients with chronic liver disease. Due to increased life expectancy and therapeutic advances in chronic liver disease, the importance of managing osteoporosis and other bone diseases in patients with chronic liver disease is expected to increase. Consequently, specific guidelines need to be established in the near future.

A novel nonosteocytic regulatory mechanism of bone modeling

Osteocytes, cells forming an elaborate network within the bones of most vertebrate taxa, are thought to be the master regulators of bone modeling, a process of coordinated, local bone-tissue deposition and removal that keeps bone strains at safe levels throughout life. Neoteleost fish, however, lack osteocytes and yet are known to be capable of bone modeling, although no osteocyte-independent modeling regulatory mechanism has so far been described. Here, we characterize a novel, to our knowledge, bone-modeling regulatory mechanism in a fish species (medaka), showing that although lacking osteocytes (i.e., internal mechanosensors), when loaded, medaka bones model in mechanically directed ways, successfully reducing high tissue strains. We establish that as in mammals, modeling in medaka is regulated by the SOST gene, demonstrating a mechanistic link between skeletal loading, SOST down-regulation, and intense bone deposition. However, whereas mammalian SOST is expressed almost exclusively by osteocytes, in both medaka and zebrafish (a species with osteocytic bones), SOST is expressed by a variety of nonosteocytic cells, none of which reside within the bone bulk. These findings argue that in fishes (and perhaps other vertebrates), nonosteocytic skeletal cells are both sensors and responders, shouldering duties believed exclusive to osteocytes. This previously unrecognized, SOST-dependent, osteocyte-independent mechanism challenges current paradigms of osteocyte exclusivity in bone-modeling regulation, suggesting the existence of multivariate feedback networks in bone modeling-perhaps also in mammalian bones-and thus arguing for the possibility of untapped potential for cell targets in bone therapeutics.

Bone Loss After Romosozumab/Denosumab: Effects of Bisphosphonates

Romosozumab and denosumab are monoclonal antibodies for the treatment of osteoporosis. Both have a rapid offset of effect resulting in loss of bone density (BMD) gained on-treatment and, in some cases, multiple vertebral fractures following treatment cessation. We recently reported disappointing results from transitioning patients from denosumab to intravenous zoledronate at the time the next denosumab injection is due. The present report re-assesses the role of bisphosphonates following the use of denosumab. In the FRAME trial, osteoporotic women were randomized to romosozumab or placebo for 1 year, then both groups were provided with open-label denosumab for the subsequent 2 years. In women completing this study at our center, we offered treatment with either oral or intravenous bisphosphonates. In the eleven women opting for intravenous treatment, zoledronate was given after a median delay of 65 days from trial-end, in the hope that this might increase skeletal uptake of the drug and, thereby, its efficacy to maintain bone density. In these women, spine BMD was 17.3% above baseline at trial-end, and still 12.3% above baseline a year later, a 73% (CI: 61%, 85%) retention of the treatment benefit. The comparable BMD figures for the total hip were 10.7 and 9.2% above baseline, a 87% (CI: 77%, 98%) retention of treatment effect. In contrast, those not receiving treatment after the conclusion of the FRAME trial lost 80-90% of the BMD gained on-trial in the following 12 months. Women treated with risedronate showed an intermediate response. In the zoledronate group, mean PINP 6 months post-FRAME was 23 ± 4 µg/L and at 12 months it was 47 ± 8 µg/L, suggesting that repeat zoledronate dosing is needed at 1 year to maintain the BMD gains. In conclusion, delaying administration of intravenous bisphosphonate when transitioning from short-term denosumab appears to increase the extent to which the gains in BMD are maintained.